Improvements to aqueous-based latex paints have resulted in their wide- scale commercial use for many end use applications. In some applications, such as for example in interior paints, aqueous-based latex paints have substantially displaced conventional non-aqueous solvent based paints.
Conventional aqueous based coatings have not, however, significantly displaced solvent-based systems as primers for use on previously painted ("repaint") surfaces and on a variety of different types of substrates due to their inability to provide the desired combination of adhesion and stain blocking that solvent based alkyd primers can provide. ln general, it is very difficuft, under wet or moist testing conditions for conventional aqueous based latex coatings to provide sufficiently adequate adhesion to old repaint alkyd painted substrates, chalky weathered paint, and wood.
In addition to adhesion, primers must have the ability to block stains. "Stainblocking" refers to the ability of a coating to prevent water soluble stains, present in or on a substrate or substrate coating, from migrating through a newly applied coating. These stains may result from the presence of certain chemicals in the substrate itself. For example, certain woods such as redwood, cedar, mahogany and the like contain tannin and lignin staining agents which are particularly concentrated in the knots and grains. If contacted by a conventional aqueous latex coating, these staining agents become extracted from the wood and migrate through the coating to the air interface. Repeated applications of the same or a similar type of aqueous latex coating will not successfully prevent these stains from reappearing on the new coating surface. Other water soluble stains which present the same type of problem include marks made by children's markers, felt or ballpoint pens and stains due to water damage on ceilings or walls. All these water soluble stains are effectively blocked by solvent based coatings in which the stain is not solubilized by the coating solvent.
While solvent based alkyd coatings have certain desirable film properties, aqueous based coatings are preferred in many ways over solvent based coatings like alkyds. One reason is due to the potential environmental, health and safety concerns caused by the organic solvents in the coating formulation. In addition, alkyd coatings are not generally as crack and flake resistant as latex based coatings, particularly in exterior applications. Furthermore, alkyds do not perform well over alkaline, cement based substrates, have poor adhesion to galvanized steel, and do not offer the easy water clean-up possible with aqueous latex coatings.
Therefore, it would be desirable if workers in the art could develop an aqueous latex coating which could be used as a substitute for solvent based alkyd coatings, such as for example as a primer coating, to provide the desirable adhesion and stain blocking properties of conventional alkyd coatings as well as the other desirable properties of conventional aqueous based latex coatings.